A magnetic resonance imaging apparatus is a medical imaging and diagnostic device which generates a magnetic resonance with atomic nuclei in an arbitrary cross-section passing horizontally through a test subject, and obtaining a tomogram in that cross-section from the magnetic resonance signal which is generated.
In order to obtain an image of high precision and high resolution, various parts of magnetic resonance imaging apparatus are being improved. In particular, in a RF coil (RF coil) which performs irradiation of electromagnetic waves and detection of a magnetic resonance signal, improvements are desired in the transmit efficiency, transmit uniformity, sensitivity and sensitivity distribution uniformity.
The QD (Quadrature Detection) method is known as a method of improving the transmit efficiency and sensitivity of an RF coil (for example, see the document, G. H. Glover, “A Comparison of Linear and Circular Polarization for Magnetic Resonance Imaging,” Journal of Magnetic Resonance, Vol. 64, and pp. 255-270 (1985)). In the QD method, a magnetic resonance signal is detected using two RF coils whose axes are disposed mutually orthogonal to each other. When this method detects a magnetic resonance signal, signals differing in phase by 90° will be detected from each RF coil. By combining these detection signals, the Signal-To-Noise ratio is theoretically improved by √2 times compared to the case where the signal is received by one RF coil. Since only ½ the power is required when irradiating the radio frequency magnetic field, the RF heating delivered to the patient is reduced. The QD method is also effective from the viewpoint of uniformity of the image obtained.
A birdcage coil is an RF coil in which transceive can be performed by the QD method (for example, see the document, Cecil E. Hayes, et al., “An Efficient, Highly Homogeneous RF Coil for Whole-Body NMR Imaging at 1.5 Tesla,” Journal of Magnetic Resonance, Vol. 63, and pp. 622-628 (1985)). In the case of a birdcage coil, transceive by the QD method are attained with one coil by arranging two feeding ports for transmitting and receiving a signal in a position in which they intersect orthogonally to each other. In the birdcage coil, the distribution of the irradiated RF magnetic field is uniform, and the sensitivity of the coil is high.
It is known that, when two feeding ports have been arranged in a birdcage coil, due to the effect of the impedance of the feeding port, the value of the capacitor disposed in the birdcage coil effectively changes, so the resonance frequency of the coil in each feeding port will change (for example, see the document, James Tropp et al., “The Theory of the Bird-Cage Resonator,” Journal of Magnetic Resonance, Vol. 82 and pp. 51-62 (1989)). It is known also that, if the capacities of one or two opposite capacitors among the plural capacitors disposed in the birdcage coil are arranged to be different, as far as concerns the RF magnetic field transmitted and received by the birdcage coil, the oscillating magnetic field generated will have a different frequency in a direction connecting a capacitor of different capacity and the capacitor in the opposite position, and the direction orthogonal to this direction (see the above-mentioned document and the document, Peter M. Joseph et al., “A Technique for Double Resonance Operation of Birdcage Imaging Coils,” IEEE Transactions on Medical Imaging, 1989, 8, pp. 286-294).
In order to operate a birdcage coil by the QD method, the RF signal supplied to the two orthogonal ports must be divided/phase-shifted, the magnetic resonance signal generated by the test subject received by two feeding ports as a circularly-polarized wave, and phase shifting/combining performed. Therefore, in the case of wiring using a birdcage coil respectively as a transmit coil and receive coil, as shown in FIG. 35, the signal line extending from the transmitter is split into two by a divider. One of the signals passes through a phase shifter, and is connected to the feeding port of the transmit coil disposed in a mutually orthogonal position, while of two signal lines extending from two feeding ports connected in orthogonal positions of the receive coil, one passes through a phase shifter to be combined into one signal line by a combiner connected to a receiver.